Capacitive fingerprint sensor and fingerprint sensing method thereof

Abstract

A capacitive fingerprint sensor is provided. The capacitive fingerprint sensor includes a sensing array, an insulating surface disposed on the sensing array, and a readout module. The sensing array includes a plurality of sensing units, and each of the sensing units includes a sensing capacitor and a reference capacitor. When a finger contacts the insulating surface, the readout module obtains a sensing time interval corresponding to the sensing capacitor of each of the sensing units and a reference time interval corresponding to the reference capacitor of each of the sensing units during a sensing period. The readout module provides a sensing output according to the sensing time interval and the reference time interval to indicate a fingerprint ridge or a fingerprint valley of the finger.

What is claimed is: 
     
       1. A capacitive fingerprint sensor, comprising:
 a sensing array, comprising a plurality of sensing units, wherein each of the sensing units comprises a sensing capacitor and a reference capacitor; 
 an insulating surface disposed on the sensing array; 
 a readout module, obtaining a sensing time interval corresponding to the sensing capacitor of each of the sensing units and a reference time interval corresponding to the reference capacitor of each of the sensing units during a sensing period when a finger contacts the insulating surface, and providing a sensing output according to the sensing time interval and the reference time interval to indicate a fingerprint ridge or a fingerprint valley of the finger; and 
 a power module, providing a sensing current to the sensing capacitor to generate a sensing voltage, and providing a reference current to the reference capacitor to generate a reference voltage, 
 wherein the readout module comprises:
 a comparing unit, comparing the sensing voltage with a bias signal to obtain a first comparison result, and comparing the reference voltage and the bias signal to obtain a second comparison result; and 
 a determining unit, obtaining the sensing output according to the first comparison result and the second comparison result, 
 
 wherein the first comparison result represents the sensing time interval corresponding to the sensing voltage, and the second comparison result represents the reference time interval corresponding to the reference voltage, wherein the sensing output represents a time difference between the sensing time interval and the reference time interval. 
 
     
     
       2. The capacitive fingerprint sensor as claimed in  claim 1 , wherein the readout module further receives the sensing voltage for generating the sensing time interval, receives the reference voltage for generating the reference time interval, and generates the sensing output according to the sensing time interval and the reference time interval. 
     
     
       3. The capacitive fingerprint sensor as claimed in  claim 1 , wherein the comparing unit comprises:
 a first comparator, having a non-inverting input terminal for receiving the bias signal, an inverting input terminal for receiving the sensing voltage, and an output terminal for providing the first comparison result; and 
 a second comparator, having a non-inverting input terminal for receiving the bias signal, an inverting input terminal for receiving the reference voltage, and an output terminal for providing the second comparison result. 
 
     
     
       4. The capacitive fingerprint sensor as claimed in  claim 3 , wherein each of the first and second comparators comprises a level shifter for respectively converting a high level of the first comparison result and a high level of the second comparison result from an analog power level into a digital power level, wherein the analog power level is greater than the digital power level. 
     
     
       5. The capacitive fingerprint sensor as claimed in  claim 1 , wherein the determining unit comprises:
 an XOR gate, generating a difference signal according to the first comparison result and the second comparison result; and 
 a counter, counting the difference signal according to a clock signal to provide the sensing output. 
 
     
     
       6. The capacitive fingerprint sensor as claimed in  claim 1 , further comprising:
 a processor, obtaining a fingerprint pattern of the finger according to the sensing output corresponding to each of the sensing units. 
 
     
     
       7. A fingerprint sensing method for a capacitive fingerprint sensor, wherein the capacitive fingerprint sensor comprises a sensing array having a plurality of sensing units and an insulating surface disposed on the sensing array, comprising:
 obtaining a sensing time interval corresponding to a sensing capacitor of each of the sensing units, and a reference time interval corresponding to a reference capacitor of each of the sensing units during a sensing period when a finger contacts the insulating surface; 
 providing a sensing output to indicate a fingerprint ridge or a fingerprint valley of the finger according to the sensing time interval and the reference time interval; 
 providing a sensing current to the sensing capacitor via a power module of the capacitive fingerprint sensor, to generate a sensing voltage; and 
 providing a reference current to the reference capacitor via the power module, to generate a reference voltage, 
 wherein the step of providing the sensing output according to the sensing time interval and the reference time interval further comprises:
 comparing the sensing voltage with a bias signal to obtain a first comparison result, by a comparing unit of the capacitive fingerprint sensor; 
 comparing the reference voltage with the bias signal to obtain a second comparison result, by the comparing unit; and 
 obtaining the sensing output according the first comparison result and the second comparison result, 
 
 wherein the first comparison result represents the sensing time interval corresponding to the sensing voltage, and the second comparison result represents the reference time interval corresponding to the reference voltage, wherein the sensing output represents a time difference between the sensing time interval and the reference time interval. 
 
     
     
       8. The fingerprint sensing method as claimed in  claim 7 , wherein the step of providing the sensing output according to the sensing time interval and the reference time interval further comprises:
 generating the sensing time interval according to the sensing voltage; and 
 generating the reference time interval according to the reference voltage. 
 
     
     
       9. The fingerprint sensing method as claimed in  claim 7 , wherein the comparing unit comprises:
 a first comparator, having a non-inverting input terminal for receiving the bias signal, an inverting input terminal for receiving the sensing voltage, and an output terminal for providing the first comparison result; and 
 a second comparator, having a non-inverting input terminal for receiving the bias signal, an inverting input terminal for receiving the reference voltage, and an output terminal for providing the second comparison result. 
 
     
     
       10. The fingerprint sensing method as claimed in  claim 9 , wherein each of the first and the second comparators comprises a level shifter for respectively converting a high level of the first comparison result and a high level of the second comparison result from an analog power level into a digital power level, wherein the analog power level is greater than the digital power level. 
     
     
       11. The fingerprint sensing method as claimed in  claim 7 , wherein the step of obtaining the sensing output according to the first comparison result and the second comparison result further comprises:
 generating a difference signal according to the first comparison result and the second comparison result; and 
 counting the difference signal according to a clock signal to provide the sensing output. 
 
     
     
       12. The fingerprint sensing method as claimed in  claim 7 , further comprising:
 obtaining a fingerprint pattern of the finger according to the sensing output corresponding to each of the sensing units.